Tailor-Made Stationary Phases

Anything that increases the column efficiency N, the column selectivity a. or the retention factor k will enhance the separation power of the column. Packed columns are characterized by low plate numbers and PCGC is therefore a low-resolution technique. The lower efficiency is compensated by the high. selectivity a of the stationary phase, and this is the main reason why so many different stationary pha.ses have been developed for PCGC. Capillary columns on the other hand have very high plate numbers and, therefore, the number of stationary phases can be restricted because the selectivity is less important. In fact, most separation problems can be handled with four basic. stationary pha.ses and half a dozen tailor-made stationary phases. Other important features of capillary columns are their inertness and compatibility with spectroscopic detectors. In the framework of this discussion, emphasis is, therefore, on capillary columns. 

Characteristics of Optimization in Individual HPLC Modes 27.2.3.3 Tailor-Made Stationary Phases... 

Sellergren B. Molecular imprinting by noncovalent interactions tailor-made chiral stationary phases of high selectivity and sample load capacity. Chirality 1989 1 63-68. 

The chirality of DNA was applied to selective separation. A DNA aptamer as a new target-specific chiral selector for HPLC was investigated by Michaud et al. [119,120]. They showed that a tailor-made chiral stationary phase based on a DNA aptamer with known stereospecific binding for the D-enantiomer of the oligopeptide, arginine-vasopressin, exhibits enantioselectivity between the d- and L-peptides. This DNA-based target-specific aptamer chiral stationary phase provides a powerful tool for the resolution of small (bioactive) molecule enantiomers. 

The nature of the counter ion is important because it establishes the exact value of the potential difference between the stationary phase and the bulk eluent, and also, if it exhibits suitable absorption characteristics, indirect photometric detection of UV-inactive analytes is feasible. Moreover, the choice of the counter ion of the potential determining ion allows a tailor-made separation of the analytes since adsorbophilic counter ions may compete with the analyte for interaction with the potential determining ion, thereby decreasing analyte retention. Different counter ions may alter the elution sequences of a series of analytes with potential advantage for resolution and identification purposes [143]. 

Chemistry and material sciences are key disciplines for the development of advanced and more specific adsorbents. The stability and reproducibility of chromatographic columns was significantly increased by the introduction of spherical instead of irregular stationary phases. Recently, another step forward was made by the development of high efficiency monolithic columns with a rather low pressure drop. Future, further improvements, which include surface activation and internal pore structures of stationary phases, should help to tailor stationary phases for certain applications. But, besides the need for more specific and efficient solid phases, their cost is often a major problem for the widespread application of preparative chromatography. 

Use of MIPs as the tailor made sorbent for solid phase extraction (SPE) in selective enrichment, separation or pre-concentration of pharmaceuticals from the complex natural matrices is one of the important applications of the MIPs. MIPs are widely used as the stationary phase for analytical racemic separations of drugs. MIPs also find the use as in vitro controlled/sustained drug delivery devices, although its application in this field is just at a budding stage. 

Stationary phases in capillary SFC [30] are usually based on a polysiloxane backbone, cross-linked by means of a free-radical initiator, and methyl, phenyl, octyl, biphenyl, and cyanopropyl substituents are incorporated into them. These polysiloxane phases exhibit wide ranges between their glass transition and decomposition temperatures, and this feature sets them apart from most high molecular-mass polymers, as they allow useful operating conditions for chromatography. Polysiloxanes can be tailor-made for specific... 

For a given sample and separation mechanism, the separation selectivity of the chromatographic system depends on the nature of both mobile as well as stationary phase. Interactions of solute with mobile and stationary phases may include a mixture of non-polar dispersive and various polar forces. The terms polar and non-polar have been commonly used to describe a property of both the solute, as well as mobile and stationary phases. These terms, however, should not be confused with selectivity. The separation selectivity of a stationary phase can be changed discontinuously by selection of a proper column packing (by its polarity or other parameters), or tuned continuously, synthesizing tailor-made phases or mixing stationary phases differing in polarity. ... 

Gas-solid chromatography comprises the techniques with an active solid as the stationary phase. Separation depends on differences in adsorption of the sample components on inorganic adsorbents (i.e., silica, alumina, carbon black) or on organic adsorbents such as styrene-divi-nylbenzene copolymers. Separation can also occur by a size exclusion mechanism, such as the separation of gases on synthetic zeolites or molecular sieves. GSC is performed on packed columns or on open tubular columns on the walls of which a thin layer of the porous material is deposited [porous layer open tubular (PLOT) columns], GSC nowadays is used only for special separation problems. and GSC columns are, therefore, referred to as tailor-made columns,... 

The functionalities in those phases are the methyl, phenyl, cyano, trifluoro, and hydroxy-ether group, respectively. The similarity with HPLC phases is remarkable. Based on the above, Sandra et al. [22] proposed five basic phases for capillary GC. The formulas are given in Figure 12. For the majority of high-resolution separations, these stationary phases provide more than adequate performance. For some applications, the functionalities are combined on the same siloxane backbone, or columns are coupled (selectivity tuning). Functionalities can also be modified to provide specific interactions. Optical phases and liquid crystals complete the set of preferred CGC phases. These tailor-made phases are discussed in Section ] 1.3.4.3.3. 

In laboratory practice, an affinity matrix (stationary phase) is tailor-made for the protein to be purified and filled into a chromatography column. For process applications many different affinity gels are commercially available. The raw extract is then passed through the column by using a physiological buffer as the mobile phase. In this process, the desired product is adsorbed selectively by the ligand and all unwanted components are washed away. 

Chiral separations of different /3-blockers have been performed using direct and indirect modes. Direct separation of propranolol, pindolol, nadolol, oxpren-olol, atenolol, timolol, and bupranolol has been performed using tailor-made and native chiral stationary phases. Direct resolution of propranolol, atenolol, and metoprolol has been achieved using impregnation of stationary phases with optically pure compounds such as A -(3,5-dinitrobenzoyl)-/ -(—)-a-phenylglycine and Al-(3,5-dinitrobenzoyl)-L-leucine, L-aspartic acid, L-arginine, L-lysine, and D(—) tartaric acid. 

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